Colored minerals, earths and ochers, have been used throughout human history. Natural earth minerals lend themselves to a wide range of decorations, from body paint to painting on natural or constructed walls. The colors are extremely stable, as can be seen in ancient paintings that have lasted to this day. The use of colored earth pigments is found even in the oldest civilizations.
In the scientific literature, the term Maya blue refers to a “turquoise” brilliant shade of blue that is found on murals and archaeological artifacts, for example, throughout Mesoamerica. It is described in the literature as being composed of palygorskite clay and indigo, that when mixed and heated, produce the stable brilliant blue color similar to that found in Mesoamerica. Proposed methods of preparation were performed with the intent of trying to replicate the blue color found at the historical sites and to reproduce the techniques employed by the original Maya.
H. Van Olphen, Rutherford Gettens, Edwin Littman, Anna Shepard, and Luis Torres were involved in the examination of organic/inorganic complex paint from the 1960's to the 1980's. In early studies, Littman and Van Olphen published information specifically on the synthesis of the Mayan organic/inorganic complex (Littman, Amer. Antiquity, 45:87-101, 1980; Littman, Amer. Antiquity, 47:404-408, 1982; Olphen, Amer. Antiquity, 645-646, 1966; Olphen, Science, 154:645-646, 1966). Their work did not describe the technique for making the colorant, nor explain the stability of the organic/inorganic complex. However, the results of their two decades of studies with respect to the ancient paint laid a foundation of knowledge for future investigators.
Littman synthesized indigo-attapulgite complexes and verified that his synthetic version was indistinguishable from the original pigments found in the pre-Hispanic murals and artifacts (Littman, Amer. Antiquity, 45:87-101, 1980; Littman, Amer. Antiquity, 47:404-408, 1982). The prepared samples had the same physical and chemical characteristics as the authentic Maya blue examined. Littman concluded that the remarkable stability of the attapulgite was due to the heat treatment the attapulgite received during the synthesis. Others have also synthesized compounds similar to that of Maya blue by a number of routes (Torres, Maya Blue: How the Mayas Could Have Made the Pigment, Mat. Res. Soc. Symp., 1988). They employed the Gettens test to determine whether the laboratory synthesis of Maya blue was indeed authentic with the same chemical resistant properties (Gettens, Amer. Antiquity, 27:557-564, 1962). The test was necessary because initial attempts of simply mixing the palygorskite day produced the color of Maya blue but the mixture did not possess the same chemical properties as the original organic/inorganic complex samples.
Previous literature discussions of pH pertain to the alkaline pH required to reduce the indigo prior to contacting it with the clay (Littman, Amer. Antiquity, 45:87-101, 1980; Littman, Amer. Antiquity, 47:404-408, 1982). Moreover, there was a lack of understanding regarding the chemistry for producing stable and nontoxic paint systems by combining dyes and pigments with fibrous clays. U.S. Pat. No. 3,950,180 describes color compositions that involve cationic organic basic colored compounds complexed to alkali-treated inorganic substances.
More recently, several patents and patent applications discussed indigo and related organic dyes complexed in an ionic interaction with inorganic supports. PCT Publication No. WO 01/04216 also describes ionic interactions in color compositions, wherein organic dyes undergo ion exchange with charged inorganic clays.
U.S. Pat. No. 3,950,180 covers a method of manufacturing color compositions that include zeolite and montmorillonite. U.S. Pat. No. 5,061,290 covers a method of using indigo derivatives as a dyeing agent. U.S. Pat. No. 4,246,036 covers the method of manufacturing color compositions that are comprised of asbestos-cement. U.S. Pat. No. 4,640,862 covers color compositions that are used for coating an expanded polystyrene “drop-out” ceiling tile. U.S. Pat. No. 4,868,018 covers color compositions that are used with a mixture of epoxy resin, epoxy resin hardener, and portland cement to form a coating which can be applied to a surface to form simulated marble products. U.S. Pat. No. 4,874,433 covers a method for encapsulating color compositions in and/or to a zeolite. U.S. Pat. No. 5,574,081 covers a method of manufacturing waterborne clay-containing emulsion paints with improved application performance using color compositions. U.S. Pat. No. 5,972,049 covers the method of manufacturing and using color compositions to form dye carriers used in the dyeing process for hydrophobic textiles. U.S. Pat. No. 5,993,920 covers the method of manufacturing and using color compositions with stone powder and/or cement powder, fine sawdust and/or the heart of a kaoliang stalk and other materials to form an incombustible artificial marble. U.S. Pat. No. 6,339,084 covers the method of manufacturing thiazine-indigo pigments. U.S. Pat. No. 6,402,826 covers the method and manufacturing of color compositions for paper coating.
U.S. Pat. Nos. 7,052,541 and 7,429,294 describe color compositions comprising neutral indigo derivative pigments and dyes complexed to the surface of inorganic clays. These materials are useful as paints and coatings for artistic and industrial purposes, including use in cements, plastics, papers and polymers. Upon grinding and heating the organic and inorganic component as solid mixtures or in aqueous solutions, the resulting color compositions have unprecedented stability relative to the original starting materials. U.S. Pat. No. 7,425,235 describes the use of similar starting materials in methods that rely on UV-light for preparing color compositions.
“Cool” pigmented colors appear as dark in the visible spectrum but exhibit high reflectance in the near-infrared (NIR) region of the electromagnetic spectrum. Cool pigments offer a number of benefits in a broad range of applications.                1. Incorporating cool pigments in roofing material products may enhance the life of the roof because the cool pigments reflect the NIR rays of sunlight, thus lowering the surface temperature of the roof. Cool pigments may also provide enhanced light-fastness properties by reflecting the NIR and thus reflecting more of the rays which would otherwise cause photo-degradation.        2. Using cool pigments in roofing materials may provide potential energy savings because the cool pigments reflect NIR, thus keeping the surface of the roof cooler and reducing the amount of energy needed to cool the structure under the roof.        3. Cool pigments can be used in military applications. For example, chlorophyll found in plants has high NIR reflectance. Articles coated with pigments with a spectral reflectance similar to chlorophyll would blend in with background flora. Thus, cool pigments used as vehicle coatings or textile colorants may provide cloaking. NIR night vision equipment is used to detect the reflectance of un-natural colors. Thus, if an article reflects color similar to that of chlorophyll, the person or hardware is essentially camouflaged.        
Conventional cool pigments include inorganic, heavy-metal containing components such as cobalt and chromium oxides, or mixed-metal oxides. In addition to being strategic for military purposes, metals such as cobalt and chromium pose environmental threats. Furthermore, the manufacturing processes employed to produce such metal-oxide cool pigments can produce hazardous wastes that must be safely disposed of.
Other types of conventional cool pigments include organic-based pigments such as chlorophyll, which naturally exhibit high reflectance in the NIR. However, the weatherability or light-fastness of such materials make them less than satisfactory for use in applications, such as roofing material, which is subject to degradation from sunlight exposure.
Thus, there is a need for cool pigments that exhibit excellent physical and chemical properties, are non-heavy metal based, are environmentally friendly, “green” manufactured, and can be tunable. Tunable means that the pigment can be adjusted to provide high reflectance in the NIR at a desired wavelength.
Light is composed of a select group of colors; each characterized by a specific range of wavelengths. The combination of these wavelengths in visual light differs depending on the light source. Because of this, colors often look different when compared under the influence of different light sources, such as daylight, fluorescent light, incandescent lamps, etc.
When two objects appear to match under one light source but not under another, the match is said to exhibit metamerism. Metamerism is usually discussed in terms of two illuminants (illuminant metamerism) whereby two samples may match under one light source but not under another. Metamerism is an issue for any product category where human color perceptions under different lighting conditions are important in marketing the product. For example:                Auto manufacturers often combine various parts made from different materials or materials of the same color from different suppliers. However, they need all parts of the same color to “match” whether it is sunny or cloudy outside.        Consumers buying “matching” pants and jackets under fluorescent department store lighting expect the outfit to match in sunlight as well. Even though items may come from the same manufacturer, dyes which exhibit high levels of metamerism may appear different based on differences in the light source from indoors to outdoors.        Cosmetics, particularly makeup, is another area where dyes with high metamerism may produce one look when applied using a makeup mirror lighted with incandescent light, a different look outdoors in bright sunlight and yet a different appearance in a restaurant at night.        
Thus, there is also a need for pigments that exhibit minimal metamerism.